Defining Mammals
Mammals are a group of vertebrate animals that have several defining characteristics. All mammals are endothermic, meaning they are warmblooded animals. This allows them to maintain a constant internal body temperature despite changes in the outside environment
(Britannica). Mammals are also characterized by having hair or fur covering their skin. The fur helps insulate mammals and maintain their internal body heat (Wikipedia). Female mammals have mammary glands which produce milk to feed their young. This milk contains antibodies and nutrients which help the offspring grow and fight disease (University of Hawaii). Other features common to mammals are having a larger brain compared to other animals, three middle ear bones, and a specialized diaphragm muscle that aids breathing.
Dogs are Mammals
Dogs belong to the kingdom Animalia, phylum Chordata, class Mammalia, order Carnivora, family Canidae, genus Canis, and species Canis lupus familiaris (1). This classification puts dogs firmly within the mammal category. According to the University of Hawaii, mammals are vertebrate animals that have certain distinctive characteristics (2):
- They are endothermic (warm-blooded).
- They have hair on their bodies.
- They nourish their young with milk produced by mammary glands.
- They have a neocortex region in their brains.
Dogs exhibit all of these qualities. They are able to regulate their internal body temperature. Their bodies are covered in fur. Female dogs produce milk to feed their puppies. And dogs have a large, well-developed neocortex that allows for advanced cognitive functions like emotion, learning, and memory. Therefore, it is clear that dogs can be definitively categorized as mammals.
(1) https://a-z-animals.com/blog/are-dogs-mammals/
(2) https://manoa.hawaii.edu/exploringourfluidearth/biological/mammals/what-mammal
Some Dog Breeds Lack Fur
While it is true that most dogs have fur coats, there are some breeds of dogs that are hairless or mostly hairless. Two well-known examples of hairless dog breeds are the Chinese Crested and the Peruvian Inca Orchid.
The Chinese Crested is a breed that comes in two varieties – the hairless and the powderpuff. The hairless variety is mostly hairless except for fur on the head, tail, and feet. They have soft, humanlike skin. The powderpuff variety has a full coat of fur. The Chinese Crested originated in Africa and is an ancient breed. They are lively, alert small dogs that make affectionate companions.
The Peruvian Inca Orchid is an elegant, hairless breed from Peru. They are slim, agile dogs that come in hairless and coated varieties. The hairless Peruvian Inca Orchid has wispy, sparse fur on certain parts of the body. The coated variety has short, fine fur. This is an affectionate and loyal companion dog with a graceful air about them. They do well with proper socialization and training.
Other Furless Mammals
While most mammals are covered in fur, there are some exceptions. Large mammals like elephants, rhinos, hippos and whales lack fur coats.
Elephants and rhinos have thick, hairless hides with folds and creases instead of fur. Hippos have sparse hair and rely on secreting a red oily liquid that acts as sunscreen. Whales, dolphins and other cetaceans lost their fur coats adapting to an aquatic environment. Their skin is smooth and hairless with a thick layer of blubber underneath for insulation [1].
Other marine mammals like walruses, seals and sea lions kept their fur for insulation in cold waters. Furless mammals many times have adaptations like tough hides, blubber or behaviors to compensate for the lack of an insulating fur coat [2].
Why Fur in Mammals?
Fur serves several important functions for mammals. The most well-known is insulation. Fur consists of two layers – a dense undercoat and a longer guard hair outer layer. The undercoat traps air close to the skin to retain heat. The guard hairs protect the undercoat and repel water. This dual layered coat acts as an effective insulation system to help mammals maintain their body temperature in cold environments. Some fur even changes to adapt to seasonal temperature changes, like growing thicker in the winter. Insulation from fur allows mammals to inhabit colder regions that reptiles and other furless animals could not withstand.
Fur also serves as camouflage for many mammals. Variations in fur color, pattern, and length help mammals blend into their environments. For example, Arctic foxes change their fur color from brown or gray in the summer to completely white in the winter to match the snowy terrain. The fur patterns of tigers, leopards, and jaguars feature irregular dark stripes or spots that help break up their body outline so they can stealthily stalk prey. These are just a couple examples of how mammalian fur aids in concealment and stealth.
In summary, the two main functions of fur in mammals are insulation from cold temperatures and camouflage for concealment and stealth. Fur is an essential evolutionary adaption that facilitates mammals’ survival in diverse environments.
Mammal Classification
Mammals are classified based on shared physical and behavioral characteristics. According to Wikipedia, mammals are divided into three main groups:
- Monotremata – This group includes just platypuses and echidnas which lay eggs rather than giving live birth. They have a cloaca rather than separate openings for reproduction, urination, and defecation.
- Marsupials – Females have a pouch called a marsupium where underdeveloped offspring complete their development. Examples include kangaroos, koalas, and opossums.
- Placental mammals – The fetus develops fully within the mother’s uterus and is nourished by a placenta. Most mammals like dogs, whales, bats, and humans fall in this group.
Within these groups, mammals are further divided into orders based on modes of locomotion, body structure, and habits. Some examples of orders are rodents, carnivores, cetaceans, and primates.
Exceptions to the Rules
While it’s generally true that mammals have fur or hair and animals with fur or hair are mammals, there are some interesting exceptions.
For example, there are some dog breeds that lack fur, like the Peruvian Inca Orchid and the Chinese Crested. Both of these dogs breeds are mammals as they belong to the canine species. However, due to selective breeding they lack a full coat of fur [1].
On the other side, not all animals with fur are mammals. Spiders and certain types of insects have hair-like structures on their bodies, but they are arthropods, not mammals [2].
The main difference is that the fur or hair of mammals contains sebaceous glands near hair follicles. These glands secrete oils that waterproof and maintain the fur. Arthropods lack these specialized oil glands, so their hair-like structures are not the same as mammalian fur or hair [3].
So while fur and hair are characteristic of mammals, there are exceptions. It’s important to look at other factors like anatomy, physiology, and genetics to properly classify animal species.
Mammal Evolution
Fur and other defining mammalian characteristics began developing over 300 million years ago during the Carboniferous and Permian periods. During this time, synapsids, a group of animals that includes early proto-mammals and mammal-like reptiles, started to evolve features like fur, milk-producing mammary glands, and specialized teeth.
Some of the earliest evidence of fur comes from fossilized imprints showing that cynodonts, a group of therapsids, had fur as far back as 250 million years ago. Other therapsids like gorgonopsids may have also had fur.
Over millions of years of evolution, these proto-mammals continued to develop more mammalian features leading up to the first true mammals around 225 million years ago during the Triassic period.
Mammal Diversity
According to recent research published in 2018 in the Journal of Mammalogy, the total number of known mammal species is estimated to be around 6,495 [1]. This estimate includes 96 mammal species that are now extinct. Of the remaining extant mammal species, there are approximately 6,399.[2]
This represents a significant increase from older estimates. For example, the 3rd edition of Mammal Species of the World published in 2005 listed only 5,416 mammal species (75 extinct, 5,341 extant).
Researchers attribute the increased species count to the discovery and recognition of many new species over the past decades, especially smaller mammals like rodents, bats and shrews. Advances in genetic analysis have also helped identify many new cryptic species that appear physically similar to known species but are genetically distinct.
Summary
In summary, while most mammals have fur and dogs are considered mammals, there are some exceptions to these general rules. Some breeds of dogs, like the Mexican Hairless, are born without fur coats. There are also other furless mammal species, like the naked mole rat, naked bats, and several marine mammals such as whales, dolphins and hippos.
Fur likely evolved in early mammals as a survival adaptation to regulate body temperature and protect against the external environment. However, some mammals later adapted to have less or no fur for various reasons, like marine mammals that rely on blubber for insulation. Despite some variations, mammals share key characteristics like producing milk to feed their young.
While many mammals fit the general traits of the mammal class, there are always exceptions in biology and evolution. Mammals display a remarkable diversity in morphology, physiology, and habitat. Understanding mammals requires examining both their similarities as a class and their unique adaptations as individual species.